Various types of nucleic acids which exert physiologically active functions within cells have been discovered through recent developments in biotechnology. For example, it is known that small interfering RNA (siRNA) induces degradation of mRNA of a target gene existing within cells and inhibits expression of the target gene (RNA interference). The inhibitory function against target gene expression due to the RNA interference is useful for mitigation or treatment of disease symptoms caused by irregular expression of particular genes or gene groups, and development of therapeutic agents using siRNA is expected. To utilize nucleic acids including siRNA as therapeutic agents, it is important that siRNA functions in the target cell, and therefore, it is essential to establish efficient techniques to deliver the nucleic acids into target cells.
Use of a carrier (vector) is known as a technique to deliver exogenous nucleic acid molecules or genes into cells. Vectors include virus vectors and nonvirus vectors. Virus vectors have high gene transfer efficiency; however, there are various unknown safety aspects including pathogenicity, immunogenicity and cytotoxicity. Therefore, development of safer nonvirus vectors is awaited.
As a nonvirus nucleic acid delivery carrier that promotes delivery of a nucleic acid, such as siRNA, into cells, for example, a cationic lipid with a specific structure has been reported in Patent Document 1. However, the cationic lipid reported in Patent Document 1 has a disadvantage that it shows toxicity when administered to cultured cells or living organisms. Also, Patent Document 2 discloses a composition containing an amphiphilic compound and a polycation as a carrier composition which has comparatively low toxicity and can deliver siRNA into cells. However, the composition reported in Patent Document 2 also has a safety problem since its cytotoxicity becomes non-negligible when a sufficient amount of siRNA is introduced into cells.
In light of the prior art, development of a carrier composition for nucleic acid delivery which has low toxicity and can efficiently deliver a nucleic acid, such as siRNA, into cells has been desired.                [Patent Document 1] Japanese Unexamined Patent Publication No. 2002-529439        [Patent Document 2] Japanese Unexamined Patent Publication No. 2005-508394        